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Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem. Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811's CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs). In the middle of a character, each interrupt service routine takes about 35 µs. In most cases the times are less than 25 µs which does not pose a problem. In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". The remaining "inactive" slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message. Note that the data is changed by the transmitting device one half clock cycle before it is valid. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device.

Bus Probes are a family of low-power environmental sensors with an RS485 interface. Bus Probes are very compact (measuring only 40x36mm externally) and sport an attractive look and feel. For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports. If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast. Using the primary serial port is easy. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). Data translation between different machines can be performed with ease, and applications that communicate via the primary serial port can be debugged using the secondary channel. On the other hand, the secondary serial port (Serial2) is implemented using hardware pins PA3 (input) and PA4 (output), and is controlled by the associated interrupts IC4/OC5 and OC4, respectively.

Pin 3 of PortA is the Serial2 input, and pin 4 of PortA is the Serial2 output. By setting this output LOW, the slave’s input /SS is pulled LOW. When it is low, the transceiver is in receive mode. When this bit is high, the transceiver is in transmit mode. RTU (Remote Terminal Unit) mode transmits 8-bit messages as two 4-bit hexadecimal characters in a continuous stream. The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii "name" of this particular slave). When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. After connecting as for point B, in the Settings it is possible to define a name for the device, and if the reading periodicity is less than 250 minutes, you can set an interval of hours in which the reading is not carried out / transmitted (for example you can disable the night). There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector.

Routines that temporarily disable interrupts for significant periods of time can also interfere with the Serial2 port. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. High noise immunity. We employ transient suppression and 3KV galvanic isolation. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. The rate of data transmission is expressed in bits per second, or baud. You might also consider operating the secondary serial port at a lower baud rate to relax the timing constraints. Although the maximum standard baud rate of the primary serial port is 19200 baud, nonstandard baud rates of over 80 Kbaud can be attained by the 68HC11's on-chip UART and the onboard RS232 driver. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information.

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